In this section, we expand the HHG study to a self-consistent scenario of pair production. The study is focused on using an ultraintense laser to strike a solid target [1]}, because this scheme is shown to be capable of generating abundant pair plasmas resulting in a mixed target with significant pair fraction (\(\alpha \lesssim 1\) ). We anticipate the corresponding HHG spectrum to show the distinct plasma response to the laser coupling when dense pair plasmas are present. Various effects will occur in this scenario, e.g., relativistic particle acceleration and target expansion/deformation, and these effects have significant impacts on the HHG spectrum [2]}. To have a self-consistent study on this scenario, we resort to the QED-PIC simulation with OSIRIS [3]} which accounts for the photon emission and pair production via the mBW process.
